In general, a bearing is a cylindrical piece of machinery that functions to reduce friction between two flat surfaces in contact. The simplest and most inexpensive form of bearings are plain bearings. Like all bearings, plain bearings are round and minimize points of contact within the connecting part of machinery. Hydrostatic bearings and hydrodynamic bearings are more complex forms of bearings classified as fluid bearings. They consist of rotating and stationary ring-shaped elements which rely on a thin layer of fluid, either gas or liquid, to lubricate the gap. A hydrostatic bearing is named for the constant size of its gap, while a hydrodynamic bearing is named for the varying sizes of its gap.
Both hydrostatic bearings and hydrodynamic bearings rely on pressure to generate fluid movement; however, the pressure only alters the gap size of hydrodynamic bearings. For this reason, hydrostatic bearings work well for large loads at low speeds. The fact that the gap is resistant to changes in pressure allows hydrostatic bearings to endure heavy loads without causing too great or too little fluid within the gap. Hydrodynamic bearings depend on increases in pressure to release more fluid, which means they should be used in machines that gradually gain speed when starting up to avoid generating friction by closing the gap.
The way in which fluid bearings generate their gas or liquid directly impacts their function. Hydrostatic bearings generate fluid relative to the pressure they are under because of their constant gap size. They can also be designed for either linear or rotary motion, meaning they are not restricted to functioning as wheels, but can also be beneficial in machines with complex motions such as fluid pumps. Hydrodynamic bearings function for strictly rotary motion and work well to support machines that slowly increase pressure, like steam engines and electric motors. This allows the rotating spindle to gain speed and produce lubricant.
A journal bearing is the most basic form of a hydrodynamic bearing, yet it can endure relatively heavy loads. The shaft of a journal fits in the hole of a journal bearing, which is not perfectly circular due to an average of two rectangular indents created by axial grooves on either side of the diameter. These grooves prevent horizontal displacement of the journal and allow for lubrication.
The demand for a more complex design of a journal bearing led to the shoe bearing, which is proven to be the most effective design. The design is named for the flat pads of angled surfaces on the outer end of the ring, those of which come to a spherical peak at the pivot. The pivots catch rotational motion by creating wedges, unlike older designs in which the outer circumference of the rotating element was perfectly circular. Shoe bearings last for a long period of time and require little maintenance, proving to be more reliable than previous models. However, for any speed greater than 2 m/s, hydrodynamic bearings should be used. Both hydrodynamic and hydrostatic bearings are generally small and inexpensive, but they must be maintained with appropriate lubricants.
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